Return Air Ducts In Wall Cavity

A cavity space used as a return air pathway will pull pollutants into the building from unknown sources.

Return air ducts in wall cavity. These ducts don t pose a fire hazard but will spread flame and smoke in the event of a fire. Because cavity spaces are leaky building pressure imbalances across the building envelope will occur driving air infiltration into the building. Aside from wall and floor cavities other framing cavities are also used as plenums or return air ducts. These include open floor truss cavities dropped ceilings and air handler platforms.

This article addresses the issue of unsealed wall stud cavities or panned floor joists used as a return air plenum resulting in risks such as indoor air quality issues due to negatively pressurized cavities drawing air through cracks along with any contaminants and also condensation by drawing in warm humid air. The furnace or air handler is probably on the other side of the wall behind that door. The duct layout should be considered in the initial framing design stage. For the cavity to serve as a supply or return air pathway it must contain a sealed insulated duct made of approved duct materials.

The original service call was to find. Grossly undersized return air ducts on 2 three ton ac units being replaced with larger ducts to allow proper airflow. When cavity spaces are used as return air pathways or supply air ducts a few issues will arise. Running a metal cold air return duct between studs requires knowledge of both hvac systems and the components involved in framing.

Building cavity space alone should not be used as a supply or return air pathway.